Potassium occurs in nature in mineral deposits of potassium chloride (silvite) closely associated with sodium chloride (halite), forming mechanical mixtures in the form of deposits of soluble salts (silvinite) which form “coats” or “mantles” of different proportions of potassium chloride and sodium chloride.
Mineral deposits of potassium chloride and sodium chloride normally contain other substances, such as clays and salts (calcium sulfate, magnesium sulfate, magnesium chloride), and are deep, often exceeding 1,200 meters below the surface.
The deposits are known as evaporitic deposits and constitute the most important sources of potassium salts. These salts are highly soluble in water and can easily be exploited through dissolution techniques.
One of the exploitation methods of evaporitic deposits is known as “Exploitation by dissolution”, where a solvent is injected into the “coat” of soluble salts through a first well and the brine produced is recovered through a second well.
The solvent used can be water, a diluted aqueous solution of potassium chloride, a diluted aqueous solution of sodium chloride, a diluted aqueous solution of sodium chloride and potassium chloride, or any other solution capable of dissolving, selectively or not, the potassium chloride (silvite) present in the ore.
During execution of the method, an underground cavern is developed and the shape of the cavern is controlled by injecting a water-immiscible liquid. This liquid may be, for example, a mineral oil, air, nitrogen, another inert gas, or any other fluid having a density lower than that of water at the temperature of executing the process.
The immiscible fluid creates an interface between the solvent and the cavern “ceiling” that prevents the dissolution of the ceiling and allows the cavern to grow sideways through the action of the solvent injected. Side (or horizontal) development of the cavern continues until the mineral coat is adequately mined and for as long as the cavern ceiling is stable.
Once horizontal development is exhausted, vertical development of the cavern begins. Accordingly, the injection point of solvent is raised, and the injection of immiscible fluid is controlled to stabilize the new ceiling. Hence, by way of successive horizontal “cuts” into the coat of potassium chloride to be mined, the vertical development of the cavern is carried out.
North American document no. U.S. Pat. No. 4,192,555 shows a method of exploitation of the state of the art. In this method, an aqueous solvent saturated in relation to sodium chloride and non-saturated in relation to potassium chloride is fed into an underground deposit of potassium chloride ore, such that the potassium chloride is dissolved and recovered. An insulating fluid is injected into the cavern so as to form a ceiling protection and allow the side development of the cavern and the processes of horizontal development and vertical development occur substantially as described above.
North American document no. U.S. Pat. No. 4,290,650 shows another method of exploitation of the state of the art, where two underground exploitation cavities are connected to form the cavern. During the formation of the cavities, the injection of solvent and the recovery of the brine occur through the single well associated to each cavity. Thus, each of the wells comprises a solvent input pipe and a brine output pipe.
The method of exploitation by dissolution of potassium chloride comprises two production phases: a continuous phase of “primary mining”, where the extraction of sodium chloride and potassium chloride is carried out by the continuous injection of water, and a discontinuous phase or batch mining (“secondary mining” or “selective mining”), which occurs in continuation of primary mining, and where the potassium chloride is selectively extracted, by the injection of a solution sub-saturated in potassium chloride and saturated in sodium chloride, limiting the dissolution of additional sodium chloride.
The secondary mining mainly occurs on the walls of the cavern, giving continuity to the horizontal development. The production rate (measured in tons/hour) of the selective mining is lower than the production rate of the primary mining, and is most efficient in fully matured caverns, with large exposed dissolution surfaces.
In general terms, in a cavern explored using a conventional mining method by dissolution, about 80% of the potassium chloride is extracted by primary mining, while about 20% is extracted by secondary mining.
If we consider that exploitation by dissolution is carried out horizontally, and that the mineral coat or mantle generally presents a slight slant, at the end of the primary mining there are, on the cavern ceiling, portions of pure halite with a wedge of silvinite. Since said portions remain inaccessible by the solvent due to the presence of the immiscible fluid, this wedge of silvinite (and the potassium chloride contained therein) is not exploited during the secondary mining stage.
Hence, although the method described above is broadly and commonly used, the need remains for a method of exploitation capable of increasing the percentage of extraction of potassium chloride, chiefly in relation to secondary mining.